1. Field of Invention
The present invention relates to a Cu based, sintered, sliding material. More particularly, the sliding material according to the present invention is used in a sliding bearing where corrosion-resistance against the lubricating oil or wear-resistance is required. When the opposed shaft of the sliding bearing has a rough surface and/or is made of a material such as cast iron, a part of the sliding material of a bearing, particularly graphite, is easily removed during sliding, thus forming a rough surface. The sliding material is worn out by the rough surface, which may be formed either by machining or partial removal of the material during sliding.
2. Description of Related Arts
Bronze and lead bronze, which are used in a large amount for bushes exhibit excellent wear- and load-resistance. However, along with recent change in the conditions under which sliding bearings are particularly used, such as increase in the surface pressure and rise in the temperature of lubricating oil, bearing troubles due to wear occur frequently. Hard matters are therefore added to the lead-bronze which is sintered onto a steel sheet, so as to enhance the wear-resistance such that it can meet the recent changes in conditions under which the sliding material is used.
The sliding material proposed in Japanese Examined Patent Publication No. 57-50844 filed by the present assignee is characterized in that its composition is from 10 to 40% of Pb, from 1 to 30% of hard matter, the balance being Cu; or from 10 to 40% of Pb, from 1 to 30% of hard matter, from 0.2 to 10% of Sn and/or from 0.1 to 5% of Sb, the balance being Cu. The hard matter used is Mo, Co, Fe.sub.3 P, FeB, Fe.sub.2 B, or an Ni or Co-based self-fluxing alloy having a particular composition.
It is heretofore known to add graphite, or graphite and Sn, to the Cu-based sintered alloy for use as the sliding material under non-lubricating or boundary lubricating conditions. Such a sintered alloy is disclosed in Japanese Examined Patent Publication No. 36-67, Japanese Examined Patent Publication No. 39-27985, Japanese Examined Patent Publication No. 36-13058, and Japanese Examined Publication No. 46-43681. In Japanese Examined Patent Publication No. 46-43681, the additives to the Cu-Sn based alloy or Cu-Sn-Pb based alloy are: materials having high-melting points such as metals, e.g., Zr, Cr, V, Mo, Ta and W, oxides, e.g., Al.sub.2 O.sub.3, ZrO.sub.2, TiO.sub.2, and ThO.sub.2, carbides, e.g., SiC and WC, nitrides, e.g., AlN, TaN, and TiN, borides, e.g., MoB, and WB, and silicides, e.g., CrSi.sub.2, graphite, and boron. It is proposed in this publication to use Cu alloys with these additives in various applications such as wear-resistant materials, brake materials, and the like.
The oxide, nitride and carbide proposed in Japanese Examined Patent Publication No. 46-43681 mentioned above as the hard materials, enhance the properties required for brake materials but do not achieve satisfactorily high seizure resistance under the boundary lubricating condition. Such metals as Zr, Cr, V, Mo, Ta, and W exhibit better seizure resistance than the oxide, nitride and carbide do. However, since each of these metals have difficulty forming an alloy with Cu, and even if they are somehow alloyed with Cu, their compatibility with the Cu matrix is so poor that they are easily removed from the matrix. Abrasive wear due to removed metal particles therefore occurs, which then leads the seizure.
The graphite proposed in Japanese Examined Patent Publication No. 46-43681 is held in the Cu or Cu-Sn matrix. Since the Cu-Sn matrix is strengthened by Sn, the graphite is held by the matrix more strongly than in the case where it is held in the Cu matrix. Nevertheless, the graphite may be removed from the Cu-Sn matrix during the working of the Cu-Sn alloy with the graphite additive. The graphite cannot exhibit its inherent sliding property, since the Cu or Cu-Sn matrix flows and then covers the graphite during the sliding. In this case, the wear resistance and seizure resistance are deteriorated.
Since the Pb content of the sliding material proposed in Japanese Examined Patent Publication No. 57-50844 is from 10 to 40% the compatibility is improved. However, the effect of the hard matter is not thoroughly exhibited because a portion of the hard matter included in the Pb matrix is removed from the matrix during sliding. In addition, since the Pb content is from 10 to 40%, preferential corrosion of Pb is likely to occur when the temperature of the lubricating oil is high, for example, from 120.degree. to 130.degree. C. Such corrosion occurred when the above-mentioned sliding material was used in an automatic transmission.